Russia and Iran have been the primary backers of Assad's government since the country's conflict began five years ago. The war has killed 250,000 people and displaced half the country's population.

In this photo released by the Syrian official news agency SANA, Syrian citizens gather at the scene where two blasts exploded in the pro-government neighborhood of Zahraa, in Homs province, Syria, Sunday, Feb. 21, 2016.

The main Syrian opposition coalition will decide later this week on whether to take part in peace talks scheduled to resume Wednesday in Geneva, the head of the group said Monday, while violence across the north of the Syria claimed the lives of over a dozen people, despite a partial cease-fire.

Riad Hijab, head of the Higher Negotiations Committee, acknowledged in a teleconference with journalists that Russian bombardment has decreased following the “cessation of hostilities,” which came into effect Feb. 27.

Yet he added that President Bashar Assad’s government has not released any detainees and said the flow of aid to besieged rebel-held areas was too small. These were among the main preconditions for the opposition to attend the indirect peace talks. The most recent round collapsed last month as the Syrian government launched a new ground offensive.

Hijab said that while the Russian airstrikes have become less frequent, the Syrian government has continued to attack rebels. “The Russians and the Iranians have caused a catastrophic humanitarian situation that is killing Syrians,” Hijab said.

Russia and Iran have been the primary backers of Assad’s government since the country’s conflict began five years ago. The war has killed 250,000 people and displaced half the country’s population.

An opposition-held town in northern Syria was bombed by planes earlier Monday, killing at least 12 people, monitoring groups said. The bombs struck a fuel depot in Abu Adh-Dhohour, in Idlib Province. Hijab called the attack “a massacre.”

The Britain-based Syrian Observatory for Human Rights said the bodies found in the wreckage were burned beyond recognition. It said it was too early to tell if the casualties were civilians or militants.

The Local Coordination Committees, an opposition network, put the toll at 15 dead, all civilians.

Insurgents shelled a predominantly Kurdish neighborhood in the northern city of Aleppo, killing five civilians, Syria’s state news agency SANA reported.

The next round of “proximity talks” between the government and the opposition is due to begin Wednesday in Geneva, but UN Special Envoy to Syria Staffan de Mistura has said the target is a “penciled date.” Some envoys could show up as late as March 14, he has said.

In New York, UN spokesman Farhan Haq clarified that talks would start on the afternoon of March 9 even if some participants arrive later.

Pressed on whether the HNC will participate in the negotiations, Hijab said a decision will be taken “in the coming days. Before the end of the week.”

Hijab told journalists that the opposition would not allow Assad to take any role in the 18-month transitional period envisaged in the UN roadmap. He also said that they would not accept Assad being permitted to run in future presidential elections.

He said that Assad and top officials in his government “should be tried by international tribunals.”

The cease-fire, which was sponsored by Russia and the U.S., does not include al-Qaida’s branch in Syria known as the Nusra Front and the Islamic State group.

Hijab also said that “unfortunately” all Russian airstrikes had been targeting moderate rebels. He said that a map distributed by Russia’s Defense Ministry about rebel positions was not accurate, adding that he sent a letter to UN chief Ban Ki-Moon to complain about the map.

Meanwhile, Russia has said it is aiming to step up humanitarian aid to Syria’s war-battered people by opening up its military facilities in Syria for international aid cargos.

Defense Ministry spokesman Maj. Gen. Igor Konashenkov said Monday that Russia “is ready to provide all necessary help to international and foreign organizations in the delivery of humanitarian aid.”

He said this assistance will include allowing the unloading and temporary storage of aid cargos at the Russian naval base in Tartus, as well as receiving and storing aid at the air base in Hemeimeem.

Konashenkov said Russia is also ready to provide vehicles to help deliver cargos from Tartus and Hemeimeem to Syrian regions in need.

According to the UN, a humanitarian convoy reached three areas in Kafr Batna with food and health assistance for 20,000 people on Friday. Deliveries to the remaining three areas on Monday will provide aid for more than 16,000 people.